5. Consider a damped oscillator that an object of mass m attached to a spring and submerged in a viscous liquid, as shown in the figure. The displacement of the object is x, and the spring constant is k. The resistive force resulted from the liquid is -bv, where b is a constant and v is the velocity of the object. What is the equation of motion of the object? (A) (B) (C) (D) (E)